Tcell activation

O Whether it is by the direct or indirect pathway, in order for a T cell to become activated against the transplanted organ, two interactions, or signals must take place

between the APCs and the recipient's T cells ' :

• Signal 1 is the interaction of the TCR with the foreign antigens presented by the APCs.

• A second, costimulatory signal, known as Signal 2, must also take place for T-cell activation. This signal is an interaction between one of several costimulatory receptors and paired ligands on the cell surfaces of the APCs and T cells, respectively. This interaction is of the utmost importance, as Signal 1 in the absence of Signal 2 induces T-cell anergy.

Once activated, T cells undergo clonal expansion under the influence of cytokines, specifically interleukin-2 (IL-2). These steps elicit an antidonor T-cell response that results in graft destruction.

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